The interaction of myoglobin with ligands such as 02. or CO is of obvious biological significance. However, certain details of ligand binding/dissociation are not clearly understood. Our proposal is to study these interactions under nonequilibrium conditions in order to clarify some of the mechanistic issues. The project will build on the technologies developed for studies of long range electron transfer combined with laser-pulse flash photolysis studies and the known binding affinities of the heme center. The iron(III)/iron(II) redox couple will be used as an "on/off switch" to rapidly and irreversibly control the affinity of the heme center for ligation. The switch will be activated by photoelectron transfer. The subsequent reactions will be monitored by transient absorption spectroscopy and absorption difference spectroscopy. In this way, whether ligands continually reside in the pocket or must migrate into the pocket, whether water must be expelled from the pocket to accommodate the ligand, and the influence of ligand concentration in solution on the rate of addition/ dissociation will be determined. These questions address the binding, the transport and the release of O2 and the conditions which influence the biological function of myoglobin.